Historically, digital protective relays have had an adjustable “settings” capability, which enables a protection engineer to customize the operation of a protective relay to a particular protection task in a power system. This is often a challenging exercise. For example, with a conventional overcurrent relay, the protection engineer must first determine the expected fault current and the maximum load current of the power system at the point in the system where the relay is connected. The protection engineer then establishes the operating settings of the overcurrent relay to provide a trip signal when the currents it measures from the power line reach a predetermined level above the maximum load current but below the anticipated fault current.
The determination of the anticipated fault current involves a rather complex calculation, which takes into account the source strength and voltage, the impedance of downstream transformers and the impedance of the line from the relay out to the end of the protection zone covered by the relay. In addition, the protection engineer must also often coordinate the relay which is being set with other protective relays, located both closer to the source and closer to the load than the relay being set. Further, if the line protected by the relay being set can provide power and serve load in both directions from the protective relay, the setting task becomes even more complicated.
The setting task also becomes more complicated when the protection engineer must coordinate the protection provided by the relay being set with other protective relays set by another entity, an example being when utilities connect their respective power tie lines together or when utilities connect to heavy industrial loads having privately owned generators. Such connections are typically referred to as “interties”. The relays at both ends of the intertie line portion must work together as a unit to properly protect the intertie; hence, their respective settings must be coordinated for proper operation and to prevent a malfunction, which can occur in the event of a miscalculation or misapplication of a relay at either end of the intertie.
Accordingly, it is desirable for a protective relay to be able to protect a variety of electric power arrangements and configurations without the need to calculate and apply protective relay settings. Further, it would be desirable to simplify the protection of an intertie line portion to prevent misoperation of the protection.